Cellulose is a renewable natural polymer that can be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D-glucopyranose units of the polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure. Reaction selectivity is important so that a derivative of desired chemical structure could be obtained.
Heterocyclic nitroxyl compounds are known as catalysts that participate in the selective oxidation of C-6 hydroxyl groups of cellulose molecules to aldehydes and carboxylic acids, the corresponding oxoammonium salt being known as the active direct oxidant in the reaction series. One of these chemical oxidation catalysts known for a long time is “TEMPO”, i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical. Thus, the oxidized forms of the nitroxyl radicals, N-oxoammoniumions, act as direct oxidants in the oxidation of the target cellulose molecule, whereas a main oxidant is used to bring oxygen to the reaction series and convert the nitroxyl compound back to the oxidized form.
It is known to oxidize primary alcohols to aldehydes and carboxylic acids through “TEMPO” by using sodium hypochlorite as the main oxidant (for example Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S.; J. Org. Chem. 1987, 52, 2559). To improve the yield in the oxidation of the alcohols to carboxylic acids, a mixture of sodium hypochlorite and sodium chlorate was also used (Zhao, M. M.; Li, J.; Mano, E.; Song, Z. J.; Tschaen, D. M.; Org. Synth. 2005, 81, 195).
It is also known procedure to catalytically oxidize cellulose in native cellulose fibers through “TEMPO” by using sodium hypochlorite as main oxidant (oxygen source) and sodium bromide as activator (Saito, T. et al.; Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose, Biomacromolecules 2007, 8, 2485-2491). The primary hydroxyl groups (C6-hydroxyl groups) of the cellulosic β-D-glucopyranose units are selectively oxidized to carboxylic groups. Some aldehyde groups are also formed from the primary hydroxyl groups. When the fibers of oxidized cellulose so obtained are disintegrated in water, they give stable transparent dispersion of individualized cellulose fibrils of 3-5 nm in width, that is, so-called nanofibrillar cellulose.
Selectivity of the oxidation is important so that chemicals used are not consumed to unwanted side reactions. Selectivity can be defined as ratio of carboxylic groups formed to the main oxidant consumed.
The use of sodium bromide as activator is preferred because it accelerates the reaction. For example WO01/29309 recommends touse 3 parts by weight NaBr to 4 parts of NaOCl. In the reaction series, the bromide ion acts as oxygen mediator between the main oxidant and the nitroxyl radical by oxidation to hypobromite and reduction back to bromide.
It has been presumed that high level of cellulose oxidation (such as 1.5 mmol COOH/g pulp for example), attainable by the use of sodium bromide at an optimum pH of about 10, is preferable so that the pulp can be easily disintegrated to nanofibrillar cellulose. However, in these oxidation conditions side reactions increase, which results in lowered DP (degree of polymerization) of the cellulose and consequently in lowered strength properties and gel formation ability of the nanofibrillar cellulose. In EP2216345A1, the lowering of the DP is compensated for by performing the oxidation in acidic or neutral conditions.
The use of bromine compounds in the oxidation reaction is problematic because of environmental concerns. Sodium bromide is usually used in the reaction mixture in relatively large amounts (conventionally 100-125 kg/1000 kg pulp) and it is difficult to remove bromide residues from the final cellulose product. Bromine compouds also accumulate in process waters. Further, the use of bromine in industrial scale is undesirable. Use of large amounts of sodium bromide cause corrosion problems in the equipment. Bromine compounds are generally recognized as hazardous to health, for example bromate anion which is formed as a result of side reactions is a suspected carcinogen.
Further, the optimum pH is clearly in the alkaline range when bromide-hypobromite cycle is used in the reaction, which makes the method more susceptible to side reactions.